Hydrogen is one of the most abundant elements on earth. It is found in water, and in most matter. Because it can be burned as a fuel, it has great potential as an energy carrier. However, hydrogen is rarely found in a free state. It is usually part of a compound.
To be used, hydrogen must be isolated. At this time, most hydrogen is manufactured by the steam reforming of natural gas, or partial oxidation of oil. Water electrolysis is also a well-known technology for producing hydrogen. Due to the high electrical energy needed to decompose water, however, the production cost of electrolytic hydrogen is almost three times higher than that of hydrogen derived from fossil fuels. While the large-scale hydrogen market is mainly in the fertilizer, petrochemical, metallurgical, pharmaceutical and food processing industries, there is a strong possibility of using hydrogen, in large quantities, as a clean fuel in fuel cells and gas turbines. According to some estimates, the highly expanding hydrogen market will result in a demand for hydrogen to a level of at least three times higher than the current hydrogen supply at the end of the century.
With the continued increase in costs and dwindling availability of oil and natural gas, the development of alternative techniques for hydrogen generation, using non-fossil energy sources, is of crucial importance in order to meet the anticipated enhancement in demands for hydrogen. Recently, a number of advanced concepts have been proposed for bulk hydrogen production.
Aker et al., in U.S. Pat. No. 3,616,334, produce H.sub.2 from steam, utilizing a stabilized zirconia electrolyte electrolyzer in an open cycle. There, a mixture of CO/H.sub.2 gas is used as an anode depolarizer in a solid oxide electrolyte electrolysis cell. Hydrocarbon fuel is burned to provide CO for the electrolysis cell, and the reaction product CO.sub.2 is drawn off as a waste gas. Essentially, hydrogen gas is generated through the consumption of hydrocarbon fuel. As a result, the production cost of hydrogen gas using this process is relatively high. Furthermore, the use of hazardous CO gas will make the process of doubtful acceptance for utility applications.
Brecher et al., in U.S. Pat. No. 3,888,750, produce H.sub.2 from water, utilizing aqueous sulfuric acid as the electrolyte in an electrolyzer. There, water and SO.sub.2 are supplied to the electrolyzer to produce H.sub.2 SO.sub.3. The H.sub.2 SO.sub.3 is electrochemically oxidized to form H.sub.2 SO.sub.4, while H.sub.2 is produced at the cathode. The H.sub.2 SO.sub.4 is drawn off, concentrated by evaporation, and then catalytically decomposed at about 870.degree. C. in a reduction reactor. Primary products include H.sub.2 O, SO.sub.2 and O.sub.2. The SO.sub.2 is liquified to separate it from the O.sub.2, after which the SO.sub.2 is vaporized and returned to the electrolyzer.
The cycle efficiency of the Brecher et al. system is about 45% at which the optimum concentration of H.sub.2 SO.sub.4 in the electrolyzer is about 55 wt.%. Thus, a large amount of energy must be expended in concentrating the H.sub.2 SO.sub.4 by evaporation prior to decomposition. The evaporation step is the major source of efficiency loss here. In addition, aggressive hydronium ions, H.sub.3 O.sup.+, are present during the recovering processes of SO.sub.2, causing possible corrosion problems for acid vaporizers and reduction reactors, and requiring the use of costly construction materials, such as silicon, silicon carbide, silicon nitride and silicide coated Incoloy (alloy of nickel, iron, and chromium). Also, special separators are needed in the electrolyzer design to prevent SO.sub.2 migration from the anodic compartment to the cathode, where it can be reduced to sulfur or hydrogen sulfide.
What is needed is a highly efficient method of H.sub.2 production from water or steam, utilizing a closed cycle, and eliminating H.sub.2 SO.sub.4 treatment and use of CO gas as a feed.